Reverse engineering and energetics of high frequency action potential generation

该提案侧重于理解细胞在具有挑战性和极端情况下的兴奋性，作为实验室来理解生命系统设计的策略，以优化能量和维持稳态，或动态平衡状态。除了少数例外，细胞电信号是基于一个普遍的原则：控制离子梯度通过膜的耗散，维持低细胞内Na+和细胞外K+浓度。这些梯度为电压尖峰或动作电位（ap）提供驱动力。为了维持兴奋性，离子梯度必须通过（atp燃料）Na+/K+泵的化学作用来恢复，这是一个保持细胞内稳态的过程。这种普遍的操作模式是中枢神经系统中轴突操作的基础，在弱电鱼（如本格曼鱼）中，它产生外部电流，鱼通过这种电流感知环境。在具有挑战性的条件下工作提出了能量学和维持体内平衡的策略问题。

The proposal focuses on understanding cell excitability in challenging and extreme situations as a laboratory to understanding strategies devised by living systems to optimize energetics and maintain homeostasis, or a state of dynamical equilibrium. Cellular electrical signaling is based, with few exceptions, on a universal principle: controlled dissipation of ion gradients across a membrane that sustains low intracellular Na+ and extracellular K+ concentrations. These gradients provide the driving force for voltage spikes, or action potentials (APs). To maintain excitability, ion gradients must be restored by the chemical work of (ATP-fueled) Na+/K+ pumps, a process that preserves cellular homeostasis. This universal mode of operation is the basis of operation for axons in the central nervous system, and, in weakly electric fish such as Eigenmannia, it produces external electric currents by which the fish senses its environment. Operating under challenging conditions raises issues of energetics and strategies to maintain homeostasis. Remarkably, throughout their lifetime, Eigenmannia continuously produce an oscillating dipole-like electric organ discharge (EOD) at a fixed individual specific frequency originating from columns of electrocyte cells. The species range is 200-500Hz. The organ's 1000 or so non-contractile cells are driven by releases of acetylcholine through the synaptic gap. The neural input ensures that APs fire synchronously, producing a highly regular EOD. Our first short term goal is to determine how this complex high frequency “device” produces an energy-efficient dipole oscillator. Our approach is to construct EODs from the cell up to the whole organ, remembering that understanding the cellular logistics of ion homeostasis is mandatory if the EOD is to be understood. In parallel, measurements of the EOD signal are being carried out in my collaborator's lab. The low intrinsic EOD variability has received little attention so far, but our examination promises fresh insights into EO design. The brain's input into the EO provides a high frequency signal via myelinated axons which must be robust. This connects to our second short term goal, investigating the response to sustained high frequency firing in axons not designed for that purpose, including those that have suffered mild damage. We want to understand how these myelinated neurons recover excitability and homeostasis. The weakly electric fish is a fascinating model system for studying not only the energetics but the subcellular and broader level dynamics of high frequency excitability. The study of axons subjected to sustained abnormal firing or mildly damaged will reveal homeostatic design features. Our work guided by experiment and with continuous input from several biologists ensures that the predictions are well grounded and their impact extends beyond the realms of biophysics.